1. Field of the Invention
The invention relates to the field of nitroxide mediated free radical polymerization of vaporized vinyl monomers, including acrylic acid (AAc), styrene (St), N-2-(hydroxypropyl)methacrylamide (HPMA) and N-isopropyl acrylamide (NIPAAm), on silicon wafers.
2. Description of the Prior Art
Fabrication of polymer ultrathin films with controllable surface properties is critical in many important industrial applications and academic research. As device sizes continue to shrink, the use of surface-initiated polymerization, where polymer films were directly polymerized from surface premodified initiator layers, has become increasingly important over conventional coating methods. The polymer films fabricated via the surface-initiated polymerization are end-grafted monolayers with superior chemical/mechanical stability and controllable grafting thickness and density. Typically, polymerization schemes were directly adapted from the already existed chemical synthetic schemes for their counterpart polymeric materials. However, in a surface-initiated polymerization scheme, the initiators are crowded on a two-dimensional surface and the polymerization only take place at interfaces. This inherent nature has often raised difficulty in synthesizing high molecular weight polymer products in that the effective monomer to initiator ratio near surface is lower, while the premature termination caused by impurities or side reactions in solution are more dominant than the typical polymerization where both monomers and initiators are evenly dispersed in the media. Previously, empirical methods such as adding excess initiator molecules in solution phase were proposed to improve the yield.
The prior art has described the synthesis of polymer brushes by living free radical polymerization using another similar alkoxylamine initiator, but it was performed in liquid phase. The reaction performed in liquid is quite different from that in gas phase. There have been some limited work on the synthesis of polymer (polypeptide) brushes by living polymerization using quite different initiators. Besides they used condensation polymerization techniques instead of addition polymerization in our system.
As with preparation of self-assembled monolayers (SAMs), polymer brushes are typically formed by first depositing initiating groups on a substrate surface that covalently bind thereto. Then, macromolecular chains are grown from the initiating groups using monomers that are typically similar to those traditionally used in microlithography, e.g., t-butyl acrylate. The covalent bonding of the macromolecular chains to the substrate surface opens up a number of possibilities that are not available with traditional spin-cast films. These advantages permit the use of these films in technological applications that include specialty photoresists, sensors and microfluidic networks.
A number of different approaches to synthesis of patterned polymer brushes have been described. For example, some have reported the patterning of surface bound initiators by either photoablation or photoinitiation, followed by polymerization to give discrete areas of polymer brushes, while others have detailed the growth of patterned polymer films using layer by layer techniques. In addition, a number of groups have also reported the elaboration of microcontact printed thiol monolayers to provide patterned polymer brushes.
In the past, studies have been done on graft polymerization for surface modification, but the main difficulty was the poor controlling of composition, architecture and function of the polymer layer. The appearance of living polymerization provided the chance of changing this situation. In the recent years, efforts have been made by the combination of graft polymerization and living polymerization, some have succeeded. Usually they were not highly efficient in initiating graft polymerization and applicable to various monomer systems. Most importantly, they were not good in patterning the polymer layer due to the limitation of solvent in most systems.
Ever since the nitroxide-mediated radical polymerization was proposed by M. K. Georges et al in 1994, it has been widely investigated in many polymeric systems. It is a very attractive approach to synthesize not only living homopolymers but also block-copolymers, as a result of its living characteristic. More recently, this approach has also been used to synthesize polymer from an immobilized TEMPO initiator layers at surfaces, thus creating an end-grafted polymer thin film layer as shown by Husseman, et al in 1999.